Commutating amplifier with wide dynamic range

ABSTRACT

Variable gain commutating amplifier apparatus and methods for use in a polar modulator are described. The apparatus may include two or more commutating amplifier stages configured to be switched to an output load based on a desired amplitude and/or transmit power level. The amplifier stages may include cross-coupled differential pairs to cancel RF carrier feedthrough. An additional R-2R ladder circuit may be provided to further extend the dynamic range by reducing the output power at the lowest output stages.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/019,967, entitled COMMUTATINGAMPLIFIER WITH WIDE DYNAMIC RANGE, filed on Jan. 9, 2008. Thisapplication is also related to U.S. Pat. No. 6,985,703 entitled DIRECTSYNTHESIS TRANSMITTER, issued on Jan. 10, 2006. The content of each ofthese applications is hereby incorporated by reference herein in itsentirety for all purposes.

FIELD OF INVENTION

The present invention relates generally to variable gain amplifiers usedin communication transmitters. More particularly but not exclusively,the present invention relates to a variable gain amplifier having acommutating structure configured to provide variable output power.

BACKGROUND

The signals transmitted in wireless communications systems often vary instrength and thus require the use of variable gain amplifiers in theradio transceiver. These variable gain amplifiers operate to compensatefor changing path losses in the transmitted signal.

Ideally, the variable gain amplifier (VGA) provides amplification at lownoise levels, adds little distortion, and consumes very little power.This is important because any distortion produced by the transmitterspills power into adjacent communication channels and thereby reducessystem capacity. To minimize distortion, the bias current in the VGA andother circuits is typically high—an unwanted attribute for portabledevices.

It would therefore be advantageous to have a VGA with low distortion andlow power consumption.

SUMMARY

The present invention is directed generally to apparatus and methods foruse in a polar modulator including variable gain amplifiers, based on aplurality of commutating amplifiers, for providing an adjustable outputpower level based on an amplitude modulation signal and/or power levelcontrol signal.

In one aspect, the present invention relates to a commutating amplifierapparatus for use in a polar modulator comprising a first commutatingamplifier stage coupled to a load stage and a second commutatingamplifier stage coupled in parallel with the first commutating amplifierstage and the load stage, wherein each of said first and said secondcommutating amplifier stages are configured to provide a predefinedscaled output and wherein each of said commutating amplifier stages areconfigured to be selectively switched on or off in response to aswitching signal.

In another aspect, the present invention relates to a commutatingamplifier apparatus for use in a polar modulator comprising a pluralityof commutating amplifier stages, wherein ones of the plurality ofcommutating amplifier stages are coupled to a load stage and a switchingapparatus coupled to the ones of the plurality of commutating amplifierstages, wherein said switching apparatus is disposed to switch one ormore of said plurality of commutating amplifier stages on or off inresponse to a power control signal.

In yet another aspect, the present invention relates to a method ofproviding an output signal in a polar modulator, comprising receiving,at a switching apparatus, a power control signal generating, based onthe power control signal, a plurality of switching signals and switchingon or off, based at least in part on one or more of said switchingsignals, the output of one or more of a plurality of commutatingamplifier stages coupled to an output load of the polar modulator.

Additional aspects of the present invention are described below inconjunction with the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing aspects and the attendant advantages of this inventionwill become more readily apparent by reference to the following detaileddescription when taken in conjunction with the accompanying drawingswherein:

FIG. 1 shows a simplified diagram of a polar transmitter;

FIG. 2 is a schematic of a double-balanced mixer;

FIG. 3 is a schematic of an embodiment of a commutating amplifier thatmay be used in implementations of the present invention;

FIG. 4( a) is a schematic of an embodiment of a commutating amplifiercomprising M shunt stages, in accordance with aspects of the presentinvention;

FIG. 4( b) is a schematic of an embodiment of a commutating amplifierwith switches to disable it and minimize feedthrough, in accordance withaspects of the present invention;

FIG. 4( c) is a schematic of an embodiment of a commutating amplifiercomprising M shunt stages, with only the low power stage active, inaccordance with aspects of the present invention;

FIG. 5( a) is a schematic of an embodiment of a commutating amplifierstage using cross-coupled devices to reduce feedthrough, in accordancewith aspects of the present invention;

FIG. 5( b) is a schematic of an embodiment of a commutating amplifierstage using cross coupled devices with an applied offset current, inaccordance with aspects of the present invention;

FIG. 6 is a schematic of an R-2R ladder used to scale the output powerlevel of the commutating amplifier's low power stage, in accordance withaspects of the present invention;

FIG. 7 is a schematic of a commutating amplifier having extended dynamicrange, in accordance with aspects of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a polar modulation transmitter. The transmitteroperates efficiently due to its simplicity of design, which consists ofa phase-locked loop (PLL) for providing phase/frequency modulation andan adjustable amplifier for providing amplitude/envelope modulation.

A double-balanced mixer can be used to amplitude modulate an RF carrieras shown in FIG. 2. The mixer uses a phase/frequency-modulated RFcarrier to switch the differential pairs formed by transistors N₁-N₄.This action translates the amplitude modulation signal represented bydifferential currents +I_(AM)(t) and −I_(AM)(t) to the transmitfrequency. The devices N₅-N₆ serve as cascode stages that isolate theoutput swing from the differential pair switches and the signal sources.The load consists of back termination resistance R_(L) having twocomponents of value ½R_(L), balun L₁, and tuning capacitor C₁. Thesecomponents transform the 50Ω terminal impedance to an internalresistance typically equal to 200Ω. The double-balanced mixer representspart of a direct upconversion polar transmitter.

To operate efficiently, the RF carrier signal must quickly and fullyswitch the commutating devices N₁-N₄. This action will aid in maximizingthe output of the driver withv _(out)(t)=i _(AM)(t)R _(Leff)where i_(AM)(t) is the differential current corresponding to theamplitude modulation signal and R_(Leff) is the effective load impedanceat the RF transmit frequency. The large RF signal applied to the driverresembles a square wave and generates energy at the odd-harmonics of theRF carrier, albeit with decreasing strength. With the output networktuned to the RF carrier, the output network presents a lower effectiveload impedance R_(Leff) at the higher harmonic frequencies. This aids inreducing the energy at harmonic frequencies of the RF carrier.

The required swing to fully switch the commutating devices isapproximately equal to

$V_{Sw} \geq \sqrt{\frac{\max\left( {i_{AM}(t)} \right)}{K}}$where max(i_(AM)) is the peak value of signal and K is the intrinsicgain of the MOS devices in the switching core. In practice, theparameter K varies inversely with oxide thickness t_(ox).

The driver commutates the signal current I_(AM) to produce a square-waveoutput, which can be modeled using a Fourier series as

${i_{out}(t)} = {\frac{2}{\pi}{{i_{AM}(t)}\left\lbrack {{\sin\; 2\pi\; f_{RF}t} + \frac{\sin\; 6\pi\; f_{RF}t}{3} + \ldots} \right\rbrack}}$where f_(RF) corresponds to the frequency of the RF signal, includingany phase/frequency modulation. Since the fundamental term representsthe intended transmit signal, the output signal current simply becomes

${i_{out}(t)} = {\frac{2}{\pi}{i_{AM}(t)}\sin\; 2\pi\; f_{RF}{t.}}$Moreover, the overall efficiency can be represented by

$\eta = {\frac{\left. {{RMS}\left( {i_{out}(t)} \right)} \right|_{f_{RF}}}{I_{DC}} \approx \frac{2}{\pi\rho}}$where RMS(i_(out)) equals the root-mean-squared or effective outputcurrent and ρ represents the peak-to-average ratio of the modulatingsignal i_(AM)(t). Note that the RMS value approximately equals thereciprocal of the peak-to-average ratio.

It is possible to simplify the amplitude modulation circuit shown inFIG. 2 and, in the process, improve its efficiency. Because the signali_(AM)(t) represents the amplitude of the modulated transmit signal, itwill always be positive valued. This allows the double-balanced mixer tobe reduced to a commutating amplifier as shown in the embodiment of FIG.3.

One potential benefit of this circuit is that its current consumptiontracks the signal i_(AM)(t). As a result, the efficiency of thecommutating amplifier remains relatively constant with the level ofsignal i_(AM)(t), the efficiency being given by:

$\eta = {\frac{\left. {{RMS}\left( {i_{out}(t)} \right)} \right|_{f_{RF}}}{I_{DC}} \approx \frac{2}{\pi}}$which is independent of ρ. Consequently, this implementation may providea significant overall performance advantage when compared to thedouble-balanced mixer illustrated in FIG. 2.

Although the signal i_(AM)(t) represents the amplitude or envelopevariation of the complex transmit signal, in many applications it mayalso include information related to the transmit signal's power level.This is because amplitude and power level can be conveniently combinedas follows:i _(AM)(t)→p _(Tx) ×i _(AM)(t)where p_(Tx) signifies the designated power control level. In practice,the amount of power control available in any circuit or system isusually limited by isolation effects.

The required power control range and ultimate system dynamic rangedepends on the application. GSM/EDGE systems rely on time divisionmultiple access (TDMA), where users alternately use the RF channel.Timing slots define when signals can be transmitted. Each transmit burstmust obey a mask that ramps the power up and down in a way thatminimizes splatter. As such, the control range for the transmitter mayapproach 50-55 dB, although typically just the top 30 dB requiresprecise settings.

In contrast, WCDMA exploits code division multiple access (CDMA) schemesthat permit users to share the same RF frequency channel. This is due tothe orthogonal spreading codes assigned to each user that make thetransmitted signals appear noise-like at the receiver. In practice, it'simportant to limit the total noise to maximize network capacity. Assuch, the network is typically configured to direct each transmitter totransmit at the power level that makes its received energy equal to theother users sharing the RF frequency channel. As a result, a typicalWCDMA transmitter must accurately control its output power from a peaklevel of +24 dBm to below −50 dBm. This amounts to at least 74 dB ofpower control.

As previously mentioned, both the commutating driver and thedouble-balanced mixer require a large RF carrier signal to quickly andfully switch their commutating devices. Furthermore, this signal mustremain fairly large even when the adjusted current i_(AM)(t) drops tolow levels (corresponding to low output power levels). At the same time,the components need to be sized to handle the operating current at fulloutput power and must be fairly large. Consequently, these devicestypically possess large capacitances that form a parasitic leakage pathfor the RF carrier signal to the RF output.

In accordance with aspects of the present invention, an alternateapproach to achieving a wide dynamic range by splitting the driverstructure into two or more stages (denoted as stages 1 through M), asillustrated in the embodiment of FIG. 4 a, may be used. This approachmay allow for removal of a large part of the coupling path at low powerlevels. To improve isolation, the off buffer stages may be disabled,their outputs may be shorted together, and the cascode devices may bebiased to ground as shown in FIG. 4 b. A power control algorithm andassociated process and hardware and/or software apparatus 410 may beused to configure and set switches S₁-S₄ according to the desiredtransmit power level, with the potential advantage of virtuallyeliminating the leakage path for any inactive circuitry. This may bebased on, for example, mapping an input power signal associated with theamplitude modulation level and/or transmit power level to a set of theswitches in order to provide a particular output power level. Forexample, at low power, only a single stage may be set to be operationalin order to improve dynamic range as is shown in FIG. 4 c, whereas oneor more of stages 1-M may be switched in or out to dynamically adjustpower level to higher levels based on the desired power and/or powerlevel signal.

With M stages, the output of the shunted driver (at the RF carrierfrequency) becomes:

${v_{out}(t)} = {\frac{2}{\pi}{\sum\limits_{M\mspace{11mu}{stages}}{{i_{AM}(t)}R_{Leff}}}}$where the summation combines the active stages feeding currentsMi_(AM)(t) to the effective output load R_(Leff). The currentsMi_(AM)(t) can be equal, linear weighted, binary weighted (i.e. 1, ½, ¼,⅛, etc.), logarithmically weighted, exponentially weighted,quadratically weighted, and/or weighted in a variety of othercombinations as are known or developed in the art. At minimum power, thecoupling factor will typically be at least 1/M^(th) the value at fulloutput power. As a potential added advantage, this topology may alsoaggressively reduce current consumption in the driver at low and evenmoderate power levels.

It is also possible to improve isolation and dynamic range by cancellingRF carrier feedthrough. This may be accomplished by adding across-coupled differential pair to one or more commutating amplifierstages as shown in FIG. 5 a. Devices N₃-N₄ are typically sized to matchdevices N₁-N₂. This creates a feedthrough path that ideally matches and(since it's cross-coupled in an opposite fashion) cancels any signalcoupled to the output by the original feedback path. In someembodiments, it may be desirable to bias the second differential pairwith an offset current, I_(offset), as shown in FIG. 5 b. It may also bedesirable to add an equal offset current to the original differentialpair consisting of transistors N₁-N₂ to avoid distortion.

To extend the driver's dynamic range even further, the lowest powerstage may tap into an R-2R ladder as shown in FIG. 6. The resistorladder steers the RF currents in a way that reduces their difference atthe output load. The RF current may connect to the ladder at one of thedesignated tap points using the cascode stages as switches. The shuntstages may connect to the top taps (+I₁ and −I₁). Connecting the lowestpower stage (or a dedicated copy of the stage) to the second taps splitsthe output currents (+I₂ and −I₂) in a way that reduces the outputcurrent by a factor of two and consequently lowers the output power by 6dB. Each lower tap on the R-2R ladder reduces the output power by anadditional 6 dB such that:

$\begin{matrix}{{\Delta\; I_{out}} = {I_{{out} +} - I_{{out} -}}} \\{= {\Delta\;{I_{N}\left( \frac{1}{2^{N - 1}} \right)}}}\end{matrix}$for an N-tap R-2R ladder, without changing the resistance seen inparallel with load resistors R_(L). In practice, the active stage may bea combination of stages that can be selected to provide finer controlthan the 6 dB steps provided by the R-2R ladder. Alternatively, thesignal current i_(AM)(t) may be scaled according to:i _(AM)(t)→p _(Tx) ×i _(AM)(t)where p_(Tx) signifies the designated power control level.

The shunt driver stages and R-2R ladder network may be combined asillustrated in FIG. 7 to expand the dynamic range of the commutatingamplifier. In addition, this approach may extend the power control rangein a very efficient manner.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

1. A commutating amplifier apparatus for use in a polar modulator,comprising: a first commutating amplifier stage coupled to a load stage;a second commutating amplifier stage coupled in parallel with the firstcommutating amplifier stage and the load stage; and a switchingapparatus, said switching apparatus configured to provide control ofsaid selective switching of said first and said second commutatingamplifier stages wherein each of said first and said second commutatingamplifier stages are configured to provide a predefined scaled outputand wherein each of said commutating amplifier stages are configured tobe selectively switched on or off in response to a switching signal;wherein each of said first and said second commutating amplifier stageshas an efficiency substantially independent of peak-to-average ratio ofan input signal coupled to each of said first and said secondcommutating amplifier stages; and wherein said switching apparatuscomprises a switching circuit configured to receive a power controlsignal associated with a desired output level and provide one or moreswitching signals to control switching of said first and said secondcommutating amplifier stages.
 2. The apparatus of claim 1 wherein saidpower control signal comprises an amplitude modulation signal.
 3. Theapparatus of claim 1 wherein said power control signal comprises anamplitude modulation signal combined with a transmit power level signal.4. A commutating amplifier apparatus for use in a polar modulator,comprising: a first commutating amplifier stage coupled to a load stage;a second commutating amplifier stage coupled in parallel with the firstcommutating amplifier stage and the load stage; a first pair oftransistors coupled to the load and a bias signal provided to the firstcommutating amplifier stage; and a second pair of transistors, coupledto the first pair of transistors, said second pair of transistorscomprising an RF switching pair coupled to an RF signal provided to thefirst commutating amplifier stages; wherein each of said first and saidsecond commutating amplifier stages are configured to provide apredefined scaled output and wherein each of said commutating amplifierstages are configured to be selectively switched on or off in responseto a switching signal; wherein each of said first and said secondcommutating amplifier stages has an efficiency substantially independentof peak-to-average ratio of an input signal coupled to each of saidfirst and said second commutating amplifier stages; and wherein saidsecond pair of transistors are further coupled to an amplitude signalprovided to the first and second commutating amplifier stages.
 5. Theapparatus of claim 4 wherein said amplitude signal is a current signal.6. The apparatus of claim 5 wherein said amplitude signal comprises anamplitude modulation signal.
 7. The apparatus of claim 5 wherein saidamplitude signal comprises an amplitude modulation signal combined witha transmit power level signal.
 8. A commutating amplifier apparatus foruse in a polar modulator, comprising: a first commutating amplifierstage coupled to a load stage; and a second commutating amplifier stagecoupled in parallel with the first commutating amplifier stage and theload stage; wherein each of said first and said second commutatingamplifier stages are configured to provide a predefined scaled outputand wherein each of said commutating amplifier stages are configured tobe selectively switched on or off in response to a switching signal;wherein each of said first and said second commutating amplifier stageshas an efficiency substantially independent of peak-to-average ratio ofan input signal coupled to each of said first and said secondcommutating amplifier stages; and wherein said first commutatingamplifier stage is configured as a cross-coupled differential pair,wherein said cross-coupled differential pair comprises: a first pair oftransistors comprising a first transistor and a second transistor, saidfirst pair of transistors coupled to the load and a bias signal providedto the first commutating amplifier stage; a second pair of transistorscomprising a first RF switching pair, said second pair of transistorscoupled to the first pair of transistors, an RF signal provided to thefirst commutating amplifier stage, and an amplitude signal provided tothe first commutating amplifier stage; and a third pair of transistorscomprising a second RF switching pair, said third pair of transistorscoupled to the first pair of transistors and the RF signal; wherein saidfirst RF switching pair and said second RF switching pair arecross-coupled to cancel signals coupled to the output by a feedthroughpath.
 9. The apparatus of claim 8 wherein said amplitude signal is acurrent signal.
 10. The apparatus of claim 9 wherein said amplitudesignal comprises an amplitude modulation signal.
 11. The apparatus ofclaim 9 wherein said amplitude signal comprises an amplitude modulationsignal combined with a transmit power level signal.
 12. The apparatus ofclaim 8 further comprising a bias circuit configured to apply an offsetcurrent to said first and said second RF switching pairs to reducedistortion.
 13. The apparatus of claim 12 wherein said bias circuitcomprises a first circuit to combine said offset current with saidamplitude signal and a second circuit to couple said offset current tosaid second RF switching pair.
 14. A commutating amplifier apparatus foruse in a polar modulator, comprising: a plurality of commutatingamplifier stages, wherein ones of the plurality of commutating amplifierstages are coupled to a load stage; and a switching apparatus coupled tothe ones of the plurality of commutating amplifier stages, wherein saidswitching apparatus is disposed to switch one or more of said pluralityof commutating amplifier stages on or off in response to a power controlsignal, and wherein one or more of said plurality of commutatingamplifier stages has an efficiency substantially independent ofpeak-to-average ratio of an input signal coupled to one or more of saidplurality of commutating amplifier stages; wherein said ones of saidplurality of commutating amplifier stages are configured to provide apredefined scaled output; wherein said switching apparatus comprises aswitching circuit configured to receive the power control signal,wherein the power control signal is associated with a desired outputpower level, and provide a plurality of switching signals to controlswitching of said one or more of said plurality of commutating amplifierstages.
 15. The apparatus of claim 14 wherein said power control signalcomprises an amplitude modulation signal.
 16. The apparatus of claim 14wherein said power control signal comprises an amplitude modulationsignal combined with a transmit power level signal.
 17. The apparatus ofclaim 14 wherein one or more of said plurality of commutating amplifierstages comprises: a first pair of transistors coupled to the load and abias signal provided to the commutating amplifier stage; and a secondpair of transistors, coupled to the first pair of transistors, saidsecond pair of transistors comprising an RF switching pair coupled to anRF signal provided to the commutating amplifier stage; wherein thesecond pair of transistors are further coupled to an amplitude signalprovided to the commutating amplifier stage.
 18. The apparatus of claim15 wherein said amplitude signal is a current signal.
 19. The apparatusof claim 18 wherein said amplitude signal comprises an amplitudemodulation signal.
 20. The apparatus of claim 18 wherein said amplitudesignal comprises an amplitude modulation signal combined with a transmitpower level signal.
 21. The apparatus of claim 14 wherein one or more ofsaid commutating amplifier stages is configured as a cross-coupleddifferential pair and wherein said cross-coupled differential paircomprises: a first pair of transistors comprising a first transistor anda second transistor, said first pair of transistors coupled to the loadand a bias signal provided to the commutating amplifier stage; a secondpair of transistors comprising a first RF switching pair, said secondpair of transistors coupled to the first pair of transistors, an RFsignal provided to the commutating amplifier stage, and an amplitudesignal provided to the commutating amplifier stage; and a third pair oftransistors comprising a second RF switching pair, said third pair oftransistors coupled to the first pair of transistors and the RF signal;wherein said first RF switching pair and said second RF switching pairare cross-coupled to cancel signals coupled to the output by afeedthrough path.
 22. The apparatus of claim 21 further comprising abias circuit configured to apply an offset current to said first andsaid second RF switching pairs to reduce distortion.
 23. The apparatusof claim 22 wherein said bias circuit comprises a first circuit tocombine said offset current with said amplitude signal and a secondcircuit to couple said offset current to said second RF switching pair.24. A method of providing an output signal in a polar modulator,comprising: receiving, at a switching apparatus, a power control signal;generating, based on the power control signal, a plurality of switchingsignals; and switching on or off, based at least in part on one or moreof said switching signals, the output of one or more of a plurality ofcommutating amplifier stages coupled to an output load of the polarmodulator, wherein one or more of said plurality of commutatingamplifier stages has an efficiency substantially independent ofpeak-to-average ratio of an input signal coupled to one or more of saidplurality of commutating amplifier stages; wherein said plurality ofcommutating amplifier stages are configured to provide a predefinedscaled output; and wherein said power control signal comprises anamplitude modulation signal.
 25. The method of claim 24 wherein saidpower control signal comprises an amplitude modulation signal combinedwith a transmit power level signal.